This invention relates to the nondestructive evaluation of the formability of metallic sheet materials such as steel, and, more particularly, to an ultrasonic technique for making such evaluations.
One of the largest tonnage uses of steel throughout the world is found in the preparing of thin flat sheets by rolling, and then forming the sheets into useful products. Most of the exterior of an automobile is made of formed steel sheets, for example. Steel sheet material is also used in siding, appliances, office equipment, containers, and a myriad of other products.
One common forming procedure is die forming, wherein the steel sheet is placed between a male die and a female die in a press. As the press closes, the sheet of steel is deformed into the shape dictated by the closed dies. Other sheet forming techniques such as bending, drawing, and ironing are also used for particular products.
Most of the steel used in sheet forming is a low carbon steel of 0.05-0.20 weight percent carbon in iron, with minor amounts of other elements present. Studies over the years have shown that such steels, as well as many other sheet materials, do not deform uniformly in sheet forming operations, due to textures created as the sheet is rolled from thicker stock. The textures result from crystallographic preferred orientations in the rolled sheet.
The nonuniform deformation of the steel sheet leads to various types of problems with the formed sheet products. In the extreme case, the steel may fail in the forming operation, such as by tearing, so that a part cannot successfully be formed. In less severe situations, there may be nonuniform thinning of the formed sheet product so that there are local thin regions or there may be nonuniform strains around the edges of the sheet known as "ears", for example.
The occurrence of these problems depends upon many factors, including the nature of the forming operation, the type of metal, melting practice for the metal, composition variations in the metal, the heat treatment of the metal, and the sequence of rolling steps used to prepare the flat sheet. While it would be desirable to determine the optimum set of conditions for these parameters and then hold them constant, such precise control is simply not possible for the very large quantities of metal used and the present state of manufacturing technology.
Consequently, various types of tests have been developed to evaluate the formability of sheet material before an attempt is made to form it. These tests generally utilize destructive measurements of mechanical properties of sheet material to determine a formability index. If the formability index is within certain allowed values, then the sheet material is judged to be acceptable for the particular forming operation and is shipped to the user for forming. On the other hand, if the formability index does not meet the required limitations, the sheet must be diverted for other, less demanding uses, or scrapped.
While the basic viability and utility of the formability index approach as a cost-saving procedure has been established, it suffers from some important shortcomings in a typical industrial setting. Presently operable techniques for measuring the formability index require that the sample being evaluated be either destroyed by mechanical testing, or at the least excised from the larger sheet in which it is embedded. This requirement is not fatal to the use of the test, but it does mean that there is a delay in obtaining test results, that it is impossible to perform "on line" measurements on the sheet as it is being rolled, and that at the least there is wasted material when samples are cut from the sheet.
There has been proposed the possibility of measuring a formability index by propagating ultrasonic waves of different types (i.e., shear waves of differing polarizations) in the plane of the sheet, but no technique has been demonstrated. It has also been proposed to propagate waves through the thickness of a sheet to determine a modulus value which is said to be related to a formability index, but again no operable method has been demonstrated.
It would be particularly desirable to have the capability to perform on line tests as the sheet is being rolled, to permit problems to be discovered without delay. Conceivably, the results of such a test could be used to actively control the rolling operation to reduce the incidence of problems caused by rolling speed, roll bites, and other controllable factors.
One possible approach to development of a nondestructive formability evaluation that could be used on line has been the discovery of crystallographically based orientation distribution coefficients, or ODCs. The ODCs can be correlated to the irregular crystallographies that cause formability problems, and can be measured nondestructively and on line with X-ray machines. The ODC approach has the shortcomings that it is heavily based in theory and therefore can be subject to the effects of erroneous assumptions of the theory, and does not take into account many of the variables known to affect sheet formability, such as composition of the sheet material. Moreover, a reflection-mode X-ray measurement samples only textures near the surface, which may not accurately reflect the character of the interior of the sheet. To sample the interior of the sheet, a transmission-mode X-ray measurement must be made, which requires a high-power X-ray source and special detectors, and can be hazardous if there is leakage of the X-rays. Although the ODC approach has some promise, as a result of these problems it has not achieved a wide degree of acceptance in the industrial community.
There remains a need for an improved approach to measuring the formability of sheet materials, preferably by a nondestructive technique that would permit on line measurement and control. The present invention fulfills this need, and further provides related advantages.